Normally, the operating cylinders of such hydraulic systems are at least partially configured in such a way that the volumes displaced by the working strokes on the high-pressure side of the operating cylinder are not equal to those on the low-pressure side. Therefore, the differential volume has to be compensated for which, in prior-art hydraulic systems, is done in that the reservoir that is needed anyway to compensate for losses due to leakage and to compensate for temperature-related volume fluctuations is dimensioned to be so large that it can accommodate these differential volumes as well. If a hydraulic system has several operating cylinders that can be actuated independently of each other, then the reservoir volume that needs to be made available can be quite considerable since the size of the reservoir has to be dimensioned taking into account the sum of all differential volumes, temperature-related fluctuations and margins for leakage. An additional problem arises if the hydraulic systems require several redundant hydraulic circuits, as is the case in aircraft construction due to safety considerations. In order to allow the use of identically designed reservoirs, the hydraulic circuit with the largest fluctuation range is the determining factor for the dimensioning of all of the reservoirs. This results in at times substantial additional requirements in terms of the reservoir volume as well as the amount of fluid for the entire system which, in turn, translates into the severe drawback that a large weight has to be carried by the aircraft.
Austrian patent application AT 401 552 B discloses a device to hold and subsequently release hydraulic fluid from a hydraulic system. This device has an operating cylinder and a feed line located between a pump and the operating cylinder so that hydraulic fluid can be fed in at high pressure at a predefined interval. Moreover, the device comprises a holding cylinder with a piston that, on one side, delimits a fluid space to hold hydraulic fluid from the operating cylinder at a discharge pressure that is considerably lower than the high pressure whereby, on the other side, the piston forms a chamber that contains gas under a low pressure and has a high-pressure cylinder with a fluid space in which a plunger piston moves, whereby the fluid space is connected to the feed line for purposes of holding and releasing hydraulic fluid at high pressure. A piston rod is arranged between the plunger piston and the piston in order to transmit movements from the plunger piston to the piston and vice versa. Furthermore, the area of the piston that is affected by the pressure of the hydraulic fluid is considerably larger than the area of the plunger piston that is influenced by the pressure of the hydraulic fluid.